Sect 8 Quiz Easy
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Questions and Answers

What effect does RNA editing have on the apo-B gene?

  • It alters the sequences of pre-mRNAs. (correct)
  • It suppresses the production of serum lipoproteins.
  • It creates a single protein isoform.
  • It reduces the expression of the apo-B gene.

    • What is the primary function of ApoB-100?

  • Binding to LDL receptors for cholesterol delivery. (correct)
  • Formation of lipoprotein complexes.
  • Catalyzing lipid synthesis.
  • Transport of glucose to cells.

    • What roles do nuclear pore complexes (NPCs) play in cellular transport?

  • They allow free diffusion of proteins larger than 40 kDa.
  • They modify mRNAs.
  • They facilitate the export of fully processed mRNAs. (correct)
  • They serve as sites for protein synthesis.

    • Which type of proteins are responsible for the structure of nuclear pore complexes?

    <p>Nucleoporins.</p> Signup and view all the answers

    What type of nucleoporins are involved in forming the annular structures of the NPC?

    <p>Membrane nucleoporins.</p> Signup and view all the answers

    How do small proteins up to 40 kDa interact with nuclear pore complexes?

    <p>They diffuse freely through nuclear pores.</p> Signup and view all the answers

    What modifications occur to the apo-B mRNA gene during RNA editing?

    <p>Conversion of CAA codon for glutamine to UAA stop codon.</p> Signup and view all the answers

    What structure forms the 'nuclear basket' associated with nuclear pore complexes?

    <p>Eight long filaments extending into the nucleoplasm.</p> Signup and view all the answers

    What process occurs first during the maturation of mRNA in eukaryotic cells?

    <p>3' Cleavage</p> Signup and view all the answers

    Which sequence is a conserved polyadenylation signal in mRNAs?

    <p>AAUAAA</p> Signup and view all the answers

    What role does the cleavage stimulatory factor (CStF) play in mRNA processing?

    <p>It stabilizes the CPSF-CStF-pre-mRNA complex.</p> Signup and view all the answers

    During polyadenylation, what protein accelerates the rate of adenine residue addition?

    <p>Nuclear poly(A) binding protein (PABPN1)</p> Signup and view all the answers

    Which of the following statements about RNA splicing is true?

    <p>It begins while transcription is ongoing for longer transcripts.</p> Signup and view all the answers

    What is the primary function of poly(A) polymerase (PAP)?

    <p>To add adenine residues to the mRNA.</p> Signup and view all the answers

    Which factor is exchanged after mRNA is transported into the cytoplasm?

    <p>PABPN1</p> Signup and view all the answers

    What is the role of the spliceosome in mRNA processing?

    <p>To remove introns and join exons.</p> Signup and view all the answers

    What happens to phosphorylated Npl3 after successful polyadenylation?

    <p>It is dephosphorylated by Glc7 nuclear phosphatase.</p> Signup and view all the answers

    What is the role of Sky1 in relation to Npl3?

    <p>Sky1 phosphorylates Npl3 causing mRNP complex dissociation.</p> Signup and view all the answers

    What is the function of a nuclear-localization signal (NLS)?

    <p>To enable recognition and transport of proteins into the nucleus.</p> Signup and view all the answers

    Which proteins interact with cargo proteins during nuclear transport?

    <p>Karyopharins such as importins and exportins.</p> Signup and view all the answers

    How do importins and exportins navigate through nuclear pores?

    <p>By using FG-repeats as temporary binding sites.</p> Signup and view all the answers

    What happens when a cargo complex reaches its destination?

    <p>The cargo complex dissociates, freeing the cargo protein.</p> Signup and view all the answers

    Which statement about nuclear-export signals (NES) is true?

    <p>Proteins with NES are recognized by receptors for nuclear export.</p> Signup and view all the answers

    What characterizes proteins that shuttle between the nucleus and cytoplasm?

    <p>They possess both a nuclear-export signal and a nuclear-localization signal.</p> Signup and view all the answers

    What is the role of RNA-binding proteins in mRNA regulation?

    <p>They bind to regulatory elements in the UTRs to control translation and degradation.</p> Signup and view all the answers

    What happens to ferritin mRNA translation at low iron levels?

    <p>IRE-BP binds to IREs, inhibiting ferritin translation.</p> Signup and view all the answers

    How does the transferrin receptor (TfR) mRNA respond to low iron concentrations?

    <p>IRE-BP binds to its 3’ UTR, preventing degradation.</p> Signup and view all the answers

    What triggers the degradation of transferrin receptor mRNA at high iron levels?

    <p>The exposure of AU-rich degradation signals in IRE.</p> Signup and view all the answers

    What is the function of ferritin in the cell?

    <p>Binds and stores iron to prevent toxic accumulation.</p> Signup and view all the answers

    What is a characteristic feature of mRNA surveillance mechanisms?

    <p>They ensure proper processing of mRNA before translation.</p> Signup and view all the answers

    Which protein is responsible for binding to the iron response elements in mRNAs?

    <p>IRE-binding protein (IRE-BP).</p> Signup and view all the answers

    What is the relationship between iron levels and the activity of IRE-BP?

    <p>Low iron levels activate IRE-BP, inhibiting ferritin translation.</p> Signup and view all the answers

    What bond is formed during the first transesterification catalyzed by the catalytic core of U6 and U2?

    <p>2’,5’ phosphodiester bond</p> Signup and view all the answers

    Which complex is responsible for rapidly degrading excised introns?

    <p>Exosomes</p> Signup and view all the answers

    What role do nuclear exonucleases play in RNA degradation?

    <p>They hydrolyze one base at a time from either end of an RNA molecule.</p> Signup and view all the answers

    What distinguishes the AU-AC introns from the usual GU-AG intron rule?

    <p>They begin and end with specific sequences AU and AC.</p> Signup and view all the answers

    What happens to antisense RNA that is transcribed in the wrong direction?

    <p>It contains polyadenylation signals that are cleaved.</p> Signup and view all the answers

    How do RNA helicases contribute to RNA degradation?

    <p>They disrupt base pairing and RNA-protein interactions.</p> Signup and view all the answers

    What is the consequence of the presence of polyadenylation signals (PAS) in the coding regions of pre-mRNA?

    <p>They may be preceded by U1 binding sites that inhibit cleavage.</p> Signup and view all the answers

    What is the function of the snRNPs in the splicing cycle of AU-AC introns?

    <p>They are involved in the cleavage and ligation steps.</p> Signup and view all the answers

    What is required for translation initiation?

    <p>Binding of eIF4E to eIF4G</p> Signup and view all the answers

    What effect does phosphorylation of CPEB have during ovulation?

    <p>It recruits poly(A) polymerase</p> Signup and view all the answers

    What is formed when PABPC1 binds to the elongated poly(A) tail and eIF4G?

    <p>A circular polysome</p> Signup and view all the answers

    What occurs when the poly(A) tail is shortened to 20 residues?

    <p>Weakened interactions with translation initiation factors occur</p> Signup and view all the answers

    In the deadenylation-dependent pathway, what happens to the deadenylated mRNA?

    <p>It can be decapped and degraded by XRN1</p> Signup and view all the answers

    Which mechanism involves decapping mRNA before it is deadenylated?

    <p>Deadenylation-independent pathway</p> Signup and view all the answers

    How are short-lived mRNAs typically characterized?

    <p>By repeated copies of an AU-rich sequence in their 3’UTR</p> Signup and view all the answers

    What role does the interaction between the 5’ cap and translation initiation factors play?

    <p>It facilitates polysome formation</p> Signup and view all the answers

    Study Notes

    Post-Transcriptional Gene Control

    • Eukaryotic pre-mRNA is processed to form mRNA through 5'-capping, 3' cleavage and polyadenylation, and RNA splicing.
    • Alternative splicing of mRNA is regulated, involving splicing factors.
    • Mechanisms for molecule transport through the nuclear pore complex are described.
    • Cytoplasmic proteins and pathways affect post-transcriptional control of mRNA.
    • Key terms include: 5' cap, alternative splicing, cleavage/polyadenylation complex, cross-exon recognition complex, FG-nucleoporins, nuclear pore complex, miRNA, mRNA surveillance, mRNP exporter, poly(A) tail, hnRNP, pre-mRNA, RNA editing, siRNA, RNA splicing, snRNA, spliceosome, SR proteins.

    Introduction

    • ~60% of human genes have alternatively spliced mRNA.
    • The text includes a quote relating to the importance of sharpening one's skills/study habits for success, and encourages further discussion if needed.

    9.1 Processing of Eukaryotic Pre-mRNA

    • Eukaryotic pre-mRNA processing occurs in the nucleus involving 5'-capping, 3' cleavage and polyadenylation, and RNA splicing.
    • 5'-cap involves addition of a 7-methylguanylate to the 5' end of pre-mRNA.
    • A capping enzyme associated with RNA polymerase II phosphorylated CTD ensures only mRNA is capped.
    • Unusual 5'-5' triphosphate bond is formed.
    • Enzymes transfer a methyl group to ribose and guanine portions.
    • Pre-mRNA associated with hnRNP proteins (heterogeneous ribonucleoprotein particles), preventing formation of short complementary intramolecular secondary structures.
    • hnRNPs are abundant RNA-binding proteins containing hnRNA.

    9.1. A 5' cap

    • Added to nascent RNA after ~25 nucleotides.
    • Composed of 7-methylGMP and methylated ribose.
    • Cap marks RNA as mRNA.
    • Removal of y-phosphate from 5' end of nascent mRNA.
    • GMP moiety transferred to 5' diphosphate on mRNA.

    9.1. B Pre-mRNA

    • Associated with hnRNP proteins (heterogeneous ribonucleoprotein particles).
    • hnRNPs are abundant RNA-binding proteins.
    • hnRNPs contain hnRNA, which includes pre-mRNA and other nuclear RNAs.
    • hnRNP proteins regulate splicing and transport of mRNA.
    • hnRNPs facilitate the hybridization of splice factors and RNA, accelerating splicing.

    9.1. C 3’ Cleavage and Polyadenylation

    • Cleavage and polyadenylation occur at a conserved segment of pre-mRNA, followed by polyadenylation.
    • Key segment typically consists of AAUAAA upstream from a polyadenylation signal site, where cleavage and polyadenylation occur.

    9.1. C Continued

    • Cleavage and Polyadenylation (CPSF) binds to AAUAAA; (CStF) binds to G/U site.
    • Cleavage and polyadenylation specificity factors (CPSF), cleavage stimulatory factors (CStF), and other cleavage factors (CFI and CFII) assist in stabilization of the complex.
    • Poly(A) polymerase (PAP) adds a string of adenosine nucleotides; PABPN1 accelerates the rate of this addition.

    9.1. D RNA Splicing

    • RNA splicing follows cleavage and polyadenylation, important for longer transcripts.
    • Splicing occurs at conserved sequences (splice sites) in intron-exon junctions in pre-mRNA.
    • ~30-40 nucleotides at each end of the intron are critical; 5’ GU and 3’ AG (or GU-AG) sequences important.
    • Splicing involves two transesterification reactions catalysed by spliceosome, joining exons and removing the intron.
    • Splicing utilizes small nuclear RNAs (snRNAs) and snRNPs (small nuclear ribonucleoprotein particles).
    • Introns are excised, forming a lariat structure, which gets degraded. Five snRNAs—U1, U2, U4, U5, and U6 —play crucial roles in mRNA splicing.

    9.1. D Continued

    • Small nuclear RNAs (snRNAs), associated with 6-10 proteins in snRNPs.
    • During splicing, snRNAs base-pair with pre-mRNA and other snRNAs, crucial for the splicing pathway.
    • ~100-200 nucleotide snRNAs; U1 snRNA binds to the 5’ splice site.

    9.1. E U2 snRNA

    • U2 snRNA binds to branch point A.
    • U2AF is a U2-associated factor associated with pyrimidine tract and 3’ AG splice site.
    • U2 snRNP associates at branch point and displaces SF1.
    • U4/U5/U6 complex forms the spliceosome with U1/U2/pre-mRNA.
    • Rearrangements in base-pairing leads to U1 and U4 release.
    • The catalytic core of U6 and U2, the first transesterification, forms the 2',5' phosphodiester bond.
    • Second transesterification connects exons and releases lariat; intron degrades.
    • Exosomes degrade the intron.
    • Nuclear exosomes are multiprotein complexes containing eleven 3'→5' exonucleases and RNA helicases.

    9.2 Regulation of Pre-mRNA Processing

    • Alternative splicing produces different mRNAs from one gene, impacting different cells.
    • Regulation occurs through RNA-binding proteins that act as repressors or activators controlling splicing at alternative sites.
    • RNA editing alters the pre-mRNA sequence to affect mature mRNA, from genomic DNA.

    9.3 Transport across the Nuclear Envelope

    • Fully processed mRNAs in nuclei remain associated with hnRNP and mRNP proteins.
    • mRNA is exported from nucleus to cytoplasm.

    9.4 Cytoplasmic Mechanisms of Post-Transcriptional Control

    • Regulation of translation using microRNAs (miRNAs): LIN-14 translation repressed by lin-4 mRNA.
    • Specific sequences in 3'UTR bind translation-control proteins.
    • Mechanisms for mRNA degradation, including deadenylation.

    9.4 Continued

    • Nonsense-mediated decay—mRNA degradation that results from premature termination codons in the mRNA sequence.
    • No-go decay—mRNA degradation for mRNA templates/unusually large structures or secondary structures up-stream from stop codons.

    9.4 Continued

    • mRNA surveillance mechanisms for protein production.
    • Prevention of export of incompletely spliced pre-mRNA.
    • Degradation of mRNA caused by skipped exons.
    • Exon skipping can include incorrect stop codons.

    9.5 mRNA localization

    • Production of proteins in specific areas in the cytoplasm.
    • Ash1 mRNA's regulation of HO gene transcription and mating type in yeast.

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    Description

    This quiz explores the intricacies of post-transcriptional gene control in eukaryotic cells. It covers essential processes such as mRNA processing, alternative splicing, and the impacts of various proteins and pathways on mRNA functionality. Key terms related to mRNA processing are included for a comprehensive understanding.

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